1. Field of the Invention
This invention relates to apparatus and methods for amplifying the power of an optical beam such as a laser beam, and more particularly to an amplification technique in which the beam to be amplified is processed in four passes through an amplification device.
2. Description of the Related Art
Numerous studies and investigations have been made toward amplifying the power of an optical beam, particularly a coherent laser beam. Applications for amplified beams include free space communications, generation of visible light by harmonic generation or Raman shifting, laser target designation, and range finding.
One recent approach to laser amplification is described in an article by Stephens, Lind and Guiliano, "Phase Conjugate Master Oscillator-Power Amplifier Using BaTiO.sub.3 and AlGaAs Semiconductor Diode Lasers", Applied Physics Letters, 50(11), 16 March 1987, pages 647-49. This system may be described as employing two-pass amplification, since the beam is transmitted twice through a diode laser structure which is employed for amplification. The laser beam to be amplified is transmitted in one direction through the diode laser amplifier, and strikes a phase conjugate mirror (PCM) on the opposite side of the power amplifier. A phase conjugated beam is retro-reflected by the PCM back through the power amplifier, where it is amplified a second time. The original beam is linearly polarized, and optical elements such as polarizing beam splitters and half-wave plates are located in the beam path so that the twice-amplified beam is deflected out of the system as an output.
While significant amplification was achieved with this system, the potential for even greater amplification exists if the beam could be processed through the power amplifier more than twice. Although this is beyond the capability of the described Stephens et al. system, a number of other systems have been disclosed in the literature which achieve four-pass amplification.
One of the four-pass systems is illustrated in FIG. 1, and was described in Natarov and Shklovskii, "Specific Configuration of a Four-Pass Laser Amplifier With a Stimulated Brillouin Scattering Mirror", Soviet Journal of Quantum Electronics 14(6), June 1984, pp. 871-72. A collimated, linearly polarized laser beam from a master oscillator (MO) 2 was passed through a polarizing beam splitter (PBS) 4 and a quarter-wave plate 6 to give the beam circular polarization, and then through a bulk Stimulated Brillouin Scattering (SBS) phase conjugate mirror 8 and power amplifier 10 in the form of a Nd:YAG laser. On the first beam pass, the power level and beam diameter were such that the intensity inside PCM 8 was below the SBS threshold, and therefore phase conjugation did not take place. After the first pass, the beam was reflected off a concave mirror 12, which reflected it back through the amplifier 10 to a focus inside the PCM 8. Since the intensity of the focused beam was above the SBS threshold, a phase-conjugate retro-beam was reflected from the PCM back through the amplifier for a third pass. The retrobeam was the phase conjugate of the original focused beam, and therefore retraced the exact path of the first and second amplifier passes, exiting from the power amplifier after a fourth pass as a collimated beam. During its last pass through the PCM, the beam diameter was once again large enough so that no SBS occurred, and the beam passed through unreflected to be coupled out of the system by PBS 4. The MO input power, and the beam sizes inside the PCM, were carefully chosen so that SBS only occurred in the focused beam after the second pass, and so that most of the power amplification was extracted on the third and fourth passes. This latter feature was employed so that any loss in the conjugation process occurred at a relatively low-power level, thereby maximizing the power extraction from the power amplifier.
The system of FIG. has two principal drawbacks. First, it would be desirable to employ a self-pumped type of PCM such as the BaTiO.sub.3 cube employed in the Stephens et al. two-pass system described above. However, the Natarov and Shklovskii approach requires the beam to pass through and into the PCM four times, with a different conjugation threshold for various passes. Implementing this approach with BaTiO.sub.3 as the conjugator would be difficult, since BaTiO.sub.3 does not have a well-defined threshold (if any). The ability to employ an optical fiber PCM would also be desirable, but using SBS in an optical fiber would also be difficult since the optical power is contained within the fiber on all passes, thus precluding the adjustment of intensities by geometrical means. SBS would occur on the fourth pass of the fiber at very high efficiency, thereby preventing any power from being coupled out of the system.
The second drawback of the FIG. 1 configuration is that it is incompatible with a diode laser power amplifier. The use of a diode laser as the amplification device is highly desirable, because of its high efficiency. However, the FIG. 1 system uses circular polarization within the power amplifier, which is incompatible with diode waveguide gain and phase-shift characteristics. Because of gain and phase shift differences between the TE and TM modes of a laser diode (polarized parallel to and perpendicular to the junction plane, respectively), the use of a circularly polarized beam coupled into the Natarov and Shklovskii power amplifier would result in a greatly altered polarization state after several passes. In general, an elliptically polarized beam composed of E field vectors of unequal amplitude would be produced, which would be very difficult to manipulate and couple out of the system.
A four-pass configuration that does not use a four-pass PCM, or circular polarization inside the power amplifier, is illustrated in FIG. 2. It was demonstrated with a Nd:YAG (Neodymium doped yttrium aluminum garnet) solid state laser as the power amplifier in the Soviet Union by Andreev et al., "Multipass Amplifier With Full Utilization of the Active Element Aperture", Soviet Journal of Quantum Electronics 13(5), May 1983, p. 641-43. A laser MO 14 produced a horizontally polarized beam which passed through a first polarizing beam splitter 16, and then through a Faraday rotator 18 which rotated the polarization 45.degree. out of the horizontal plane. A half-wave plate 20 then rotated the polarization back into the plane, after which the beam passed through a second polarizing beam splitter 22 and the power amplifier 24. A quarter-wave plate 26 and retro-mirror 28 were located behind the power amplifier to rotate the beam's polarization to vertical and reflect it back for a second pass through the power amplifier. After the second amplification pass, the second PBS 22 directed the beam into a PCM 30. Here the beam was conjugated and reflected back for a third pass through the power amplifier 24 with vertical polarization, and a fourth amplifying pass with horizontal polarization. With its horizontal polarization from the fourth pass, the beam was transmitted through the second PBS 22 to the first half-wave plate 20 and Faraday rotator 18, which rotated the polarization vertical again. With this polarization the beam was deflected by the first PBS 16 out of the system as an amplified output beam.
A variation of the Andreev et al. system was described in an article by Carr and Hanna, "Performance of a Nd:YAG Oscillator/Amplifier With Phase-Conjugation Via Stimulated Brillouin Scattering", Applied Physics B 36, 1985, pages 83-92. This system employed a loop configuration on the opposite side of the power amplifier from the PCM to reflect the beam and rotate its polarization. Referring to FIG. 3, a third polarizing beam splitter 32, half-wave plate 34 and a pair of reflectors 36 and 38 were used in place of the quarter-wave plate 26 and mirror 28 of Andreev et al.
It would be possible to use a diode laser power amplifier in the Andreev et al. and Carr and Hanna systems, since they employ linear polarization in the power amplifier and the PCM encounters the beam only once. However, with the use of a diode laser rather than the solid state lasers disclosed, there would be a serious power loss. This would occur at the start of the second pass through the power amplifier, where the beam must be coupled back into the power amplifier channel. A considerable insertion loss would be expected, with likely values of 5-7 dB, or even higher losses if there are strong aberrations in the power amplifier. This will result in reduced power extraction and lowered electrical efficiency. Thus, there is still a need for a four-pass beam amplification system that can efficiently use a diode laser as the power amplification element.
Another limitation to laser amplification is that individual power amplifiers have power limitations which may be less than the desired output beam power. In an alternate approach to achieving a greater output power, research has been conducted on the possibility of forming an aggregate output beam from monolithic arrays of lasers which are fabricated on the top surface of an AlGaAs or InGaAsP wafer. Consideration has been given to systems in which emitted light is directed perpendicular to the wafer surface by beveled mirrors etched into the surface adjacent to the emitting facets (Liau and Walpole, "Surface-Emitting GaInAsP/InP Laser With Low Threshold Current and High Efficiency", Applied Physics Letters 46(2), 15 January 1985, p. 115-17), to the use of distributed Bragg reflectors (Evans et al., "Grating Surface Emitting Laser With Dynamic Wavelength Stabilization and Far Field Angle of 0.25 Degrees", Applied Physics Letters, Vol. 49, pp. 314-15, 1986), or by having the laser optical cavity directed through the thin dimension of the laser normal to the surface (K. Iga et al., "Room Temperature Pulsed Oscillation of GaAlAs/GaAs Surface-Emitting Injection Laser", Applied Physics Letters, Vol. 45, pp. 348-50, 1984).
Since the above devices are fabricated on the top surface of substrates which are far from heat sinks, heat dissipation is poor and continuous wave (CW) operation does not appear to be feasible. The lasers in such an array may be made to operate at the same wavelength by injection-locking to an external laser, or by coupling the lasers together through waveguide arrangements on the wafer. However, no practical way to phase-lock the laser outputs has yet been devised, and it is not clear that output phase-locking could be accomplished with this type of array. Even if output phase-locking were possible, the large inactive areas between emitters would produce a highly thinned array, causing the output beam to have a very large fraction of the power in its side lobes.